1. Technical Field
This invention relates to lithographic processes for semiconductor device fabrication and energy-sensitive resist materials for use in such processes.
2. Art Background
Lithographic processes are typically employed in the manufacture of devices such as semiconductor devices. Among the lithographic processes that are available, photolithography is often utilized. Photolithographic processes have the advantage of being suitable for a blanket exposure technique. That is, a material that is sensitive to the exposing radiation is coated onto a substrate, e.g., a silicon wafer, that is being processed to form a plurality of devices. The energy-sensitive material, often referred to as a resist material, is then subjected to radiation that has been passed through a mask material so that the radiation reaching the resist delineates an image of a desired pattern therein. The pattern is then developed and transferred into the underlying substrate. Since the exposure occurs simultaneously over an entire device or a number of devices being processed on a substrate, e.g., a silicon substrate, the procedure is considered a blanket exposure.
A blanket exposure procedure is advantageous because it is relatively fast compared to other methods such as the raster scan technique that is used when the exposing radiation is a beam of electrons. However, generally, the precision of the pattern, referred to as resolution, that is obtained through a blanket exposure is reduced as the wavelength of the exposing radiation decreases. Because the trend in semiconductor devices is toward patterns with smaller features (0.5 .mu.m to 0.35 .mu.m to 0.25 .mu.m to 0.18 .mu.m), the wavelength of the exposing radiation must decrease in order to remain less than the size of the pattern features. Consequently, resist materials that are sensitive to radiation at these smaller wavelengths (i.e. wavelengths less than 300 nm), and in which patterns with acceptable resolution are developed, continue to be sought.
One class of resist materials in which acceptable pattern resolution is obtained when the wavelength of the exposing radiation is between 220 nm and 365 nm contain polymers of hydroxystyrene in which at least a portion of the hydroxyl (OH) functionality is protected by moieties which cleave from the polymer in the presence of acid. These polymers contain the following moieties: ##STR1## wherein Z denotes the acid sensitive moiety. These polymers are described in U.S. Serial No.08/767,493 filed Dec. 16, 1996 to Houlihan et al., which is hereby incorporated by reference.
Such polymers are combined with an energy sensitive material such as a photoacid generator (PAG) and other materials to form the resist material. When the resist material is exposed to radiation, the PAG generates an acid moiety and the Z substituent is cleaved and replaced by a hydrogen (H) atom. Thus, upon exposure to patterned radiation, there is a chemical difference between the altered polymer in the exposed region and the unaltered polymer in the unexposed region of the resist material. This chemical difference is exploited to develop a pattern that corresponds to the patterned radiation.
Although the above-described polymers have been demonstrated as useful in lithographic processes for device fabrication in which the wavelength of the exposing radiation is 248 nm, some improvement in the performance of these polymers is sought. Specifically, in order for these polymers to have a glass transition temperature (T.sub.g) such that the resist material does not flow when subjected to elevated temperatures during processing, the polymers must be cross-linked to a certain extent. Typically, the number of cross-linked units in a polymer is about two to three mole percent of the units that make up the polymer. One known cross-linking unit is hydrogenated bisphenol A. Although these cross-linked polymers have an acceptable T.sub.g, it is difficult to consistently produce polymers with the same degree of cross-linking. Resist polymers with moieties that hydrogen bond upon deprotection to increase the T.sub.g of the polymers upon deprotection are described in U.S. Pat. No. 4,939,070 to Brunsvold et al. However, the degree of hydrogen bonding and, thus, the Tg of the polymer, is dependent on the extent of deprotection and is therefore, difficult to control and to consistently reproduce. Therefore, a method is desired for controllably producing a resist polymer with a desirable T.sub.g that does not require cross-linking.